First slide

Power

Question

An engine accelerates a car of mass 800 kg to a speed of 72 km/h. If the frictional force is 10 N per ton, the power developed by the engine is:

Easy

Solution

Speed of car, v = 72 km/h = 72 $\times \frac{5}{18} = 20 {ms}^{- 1}$
KE = $\frac{1}{2} {mv}^{2} = \frac{1}{2} \times 800 \times 400 = 400 \times 400 J$
$P = \frac{KE}{time} = \frac{400 \times 400}{32} = 5000 W = 5 kW$

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